Inks for 3D printing having low print through depth

ABSTRACT

In one aspect, inks for use with a three-dimensional (3D) printing system are described herein. In some embodiments, an ink described herein comprises up to 80 wt. % oligomeric curable material; up to 80 wt. % monomeric curable material; up to 10 wt. % photoinitiator; up to 1 wt. % non-curable absorber material; and up to 10 wt. % one or more additional components, based on the total weight of the ink, and wherein the total amount of the foregoing components is equal to 100 wt. %. Additionally, the photoinitiator is operable to initiate curing of the oligomeric curable material and/or the monomeric curable material when the photoinitiator is exposed to incident curing radiation having a peak wavelength λ. Moreover, the ink has a penetration depth (D p ), a critical energy (E c ), and a print through depth (D PT ) at the wavelength λ of less than or equal to 2×D p .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/182,926, filed Nov. 7, 2018, which claims priority pursuant to 35U.S.C. § 119 to U.S. Provisional Patent Application No. 62/582,631,filed Nov. 7, 2017, each of which is hereby incorporated by reference inits entirety.

FIELD

The present invention relates to inks for use with three-dimensional(3D) printing systems.

BACKGROUND

Some commercially available 3D printers or additive manufacturingsystems, such as the ProJet™ 3D Printers manufactured by 3D Systems ofRock Hill, S.C., use inks, which are also known as build materials, thatare jetted through a print head as a liquid to form various 3D objects,articles, or parts. Other 3D printing systems also use an ink that isjetted through a print head or otherwise dispensed onto a substrate. Insome instances, the ink is solid at ambient temperatures and converts toliquid at elevated jetting temperatures. In other instances, the ink isliquid at ambient temperatures. Moreover, in some cases, the ink can becured following dispensing and/or deposition of the ink onto thesubstrate. Curing can be achieved using a laser or other source ofelectromagnetic radiation.

Other 3D printers form 3D articles from a reservoir, vat, or containerof a fluid ink or build material or a powdered ink or build material. Insome cases, a binder material or a laser or other source is used toselectively solidify or consolidate layers of the ink or build materialin a stepwise fashion to provide the 3D article.

In 3D printing systems using curing radiation, the curing radiation canpenetrate deeper into the ink than intended or desired. Morespecifically, the radiation can penetrate deeper than the portion of inkthat is intended to be cured or consolidated as part of the printedarticle structure. Such an undesired, excess cure depth can be referredto as “print through” or “print through depth.” The occurrence of printthrough can be problematic for a number of reasons. First, print throughcan result in the formation of an undesired “gummy” layer of partiallycured ink or build material on certain surfaces of an additivemanufacturing system (such as one or more “down surfaces”). Second,print through wastes build material. Third, even at its most benign,print through generally requires compensation in the build process totake into account that some layer or other of the printed article willbe different than intended (e.g., different than a correspondingcomputer aided design or “CAD” file dictates). For example, suchdeviations can sometimes be accounted or compensated for when creatingor selecting a specific CAD file to be used to form a printed article.However, such compensation may not be accurate, leading to partdistortion and general loss of printing accuracy. Finally, theoccurrence of print through generally introduces a greater number ofunknown or imprecise values into a build process. Moreover, the greaterthe print through, the greater the introduction of error and/oruncertainty. Such uncertainly is of course undesired in an additivemanufacturing process.

Therefore, there exists a need for improved methods and inks for 3Dprinting that have improved print through properties.

SUMMARY

In one aspect, inks for use with a 3D printer are described herein,which, in some embodiments, may offer one or more advantages over priorinks, particularly radiation-curable inks for use in additivemanufacturing. For example, inks described herein can be used to printarticles with improved accuracy and/or precision. Inks described hereincan also reduce the amount of waste associated with an additivemanufacturing process. Inks described herein, in some cases, alsoprovide one or more of the foregoing advantages without sacrificingspeed of the additive manufacturing process, without sacrificing energyefficiency of the additive manufacturing process, and/or withoutsacrificing desired mechanical properties of the printed articles.Moreover, inks described herein can be used in a variety of different 3Dprinters or additive manufacturing systems, such as those based onStereolithography (SLA), Digital Light Processing (DLP), and Multi-JetPrinting (MJP).

In some embodiments, an ink for use in a 3D printing system describedherein comprises up to 80 wt. % oligomeric curable material; up to 80wt. % monomeric curable material; up to 10 wt. % photoinitiator; up to10 wt. % non-curable absorber material; and up to 10 wt. % one or moreadditional components, based on the total weight of the ink. It is to beunderstood, of course, that the total amount of the oligomeric curablematerial, monomeric curable material, photoinitiator, non-curableabsorber material, and one or more additional components is equal to 100wt. %. The one or more additional components may include a colorant, aninhibitor, and/or a stabilizing agent.

Additionally, the photoinitiator of an ink described herein is operableto initiate curing of the oligomeric curable material and/or themonomeric curable material when the photoinitiator is exposed toincident curing radiation having a Gaussian distribution of wavelengthsand a peak wavelength λ. Moreover, the ink has a penetration depth(D_(p)) and a critical energy (E_(c)) at the wavelength λ. The termsD_(p) and E_(c) are described in further detail below. The ink also hasa print through depth (D_(PT)) at the wavelength λ of less than or equalto 2×D_(p), or less than or equal to 1.5×D_(p). The term D_(PT) isdescribed in further detail below. Moreover, in some cases, an inkdescribed herein has a D_(p) value and an E_(c) value at the wavelengthλ that correspond to such a D_(PT) value. For instance, in someembodiments, an ink described herein has a ratio of D_(p) to E_(c), inunits of (μm cm²)/mJ, of at least 10 or at least 15. In some cases, theratio of D_(p) to E_(c), in units of (μm cm²)/mJ, is between 10 and 50,between 10 and 25, between 10 and 15, between 10 and 13, between 15 and50, between 15 and 30, between 15 and 25, or between 19 and 25. Asdescribed in more detail below, it is believed that inks having suchproperties can provide improved printing performance, including due toimproved interaction with incident curing radiation.

In some exemplary embodiments, an ink described herein has a D_(p) of60-100 μm and an E_(c) of 2-4 mJ/cm². In other instances, the D_(p) ofthe ink is 101-150 μm, and the E_(c) of the ink is 4-20 mJ/cm². In stillother cases, the D_(p) of the ink is 151-200 μm, and the E_(c) of theink is 8-15 mJ/cm².

As described further below, the amounts of photoinitiator and/ornon-curable absorber material included in an ink can be selected toobtain a desired D_(p), E_(c), and/or D_(PT) value, in combination withother components of the ink. In some embodiments, for example, an inkdescribed herein comprises up to 5 wt. % photoinitiator and up to 2 wt.% or up to 1 wt. % non-curable absorber material. Additionally, in someinstances, the total absorbance of the non-curable absorber material atthe wavelength λ is about 0.1 to 10 times the total absorbance of thephotoinitiator at the wavelength λ. Further, in some cases, both thenon-curable absorber material and the photoinitiator of an ink describedherein have an absorption peak within 30 nm of the wavelength λ.

In another aspect, methods of forming a 3D article by additivemanufacturing are described herein. In some embodiments, such a methodcomprises providing an ink described herein and selectively curing aportion of the ink using incident curing radiation having a Gaussiandistribution of wavelengths and a peak wavelength at the wavelength λ.For example, in some instances, the ink has a print through depth(D_(PT)) at the wavelength λ of less than or equal to 2×D_(p), and/or aratio of D_(p) to E_(c), in units of (μm cm²)/mJ, between 10 and 50.Additionally, in some embodiments of a method described herein, the inkis selectively cured according to preselected computer aided design(CAD) parameters, and the D_(p) corresponds to a voxel depth of the CADparameters.

Moreover, in some cases, providing the ink comprises selectivelydepositing layers of the ink in a fluid state onto a substrate to formthe three-dimensional article. Alternatively, in other embodiments,providing the ink comprises retaining the ink in a fluid state in acontainer, and selectively curing a portion of the ink comprisesselectively applying the curing radiation to the ink in the container tosolidify or consolidate at least a portion of a first fluid layer of theink, thereby forming a first solidified or consolidated layer thatdefines a first cross-section of the article. Such a method may alsofurther comprise raising or lowering the first solidified layer toprovide a second fluid layer of the ink at a surface of the fluid ink inthe container, and selectively applying the curing radiation to the inkin the container to solidify at least a portion of the second fluidlayer of the ink, thereby forming a second solidified layer that definesa second cross-section of the article, the first cross-section and thesecond cross-section being bonded to one another in a z-direction. Asdescribed further hereinbelow, the foregoing steps may be repeated anydesired number of times needed to complete the 3D article.

In still another aspect, printed 3D articles are described herein. Suchan article can be formed from any ink and using any method describedherein. Such printed 3D articles, in some cases, have superior accuracycompared to some other 3D articles.

These and other embodiments are described in greater detail in thedetailed description which follows.

DETAILED DESCRIPTION

Embodiments described herein can be understood more readily by referenceto the following detailed description and examples. Elements, apparatusand methods described herein, however, are not limited to the specificembodiments presented in the detailed description and examples. Itshould be recognized that these embodiments are merely illustrative ofthe principles of the present disclosure. Numerous modifications andadaptations will be readily apparent to those of skill in the artwithout departing from the spirit and scope of the disclosure.

In addition, all ranges disclosed herein are to be understood toencompass any and all subranges subsumed therein. For example, a statedrange of “1.0 to 10.0” should be considered to include any and allsubranges beginning with a minimum value of 1.0 or more and ending witha maximum value of 10.0 or less, e.g., 1.0 to 5.3, or 4.7 to 10.0, or3.6 to 7.9. Similarly, a stated range of “1 to 10” should be consideredto include any and all subranges beginning with a minimum value of 1 ormore and ending with a maximum value of 10 or less, e.g., 1 to 5, or 4to 10, or 3 to 7, or 5 to 8.

All ranges disclosed herein are also to be considered to include the endpoints of the range, unless expressly stated otherwise. For example, arange of “between 5 and 10,” “from 5 to 10,” or “5-10” should generallybe considered to include the end points 5 and 10.

Further, when the phrase “up to” is used in connection with an amount orquantity, it is to be understood that the amount is at least adetectable amount or quantity. For example, a material present in anamount “up to” a specified amount can be present from a detectableamount and up to and including the specified amount.

The terms “three-dimensional printing system,” “three-dimensionalprinter,” “printing,” and the like generally describe various solidfreeform fabrication techniques for making three-dimensional articles orobjects by stereolithography, selective deposition, jetting, fuseddeposition modeling, multi-jet modeling, and other additivemanufacturing techniques now known in the art or that may be known inthe future that use a build material or ink to fabricatethree-dimensional objects.

I. Inks for 3D Printing

In one aspect, inks for use with a 3D printer are described herein. Insome embodiments, an ink described herein comprises up to 80 wt. %oligomeric curable material, up to 80 wt. % monomeric curable material,up to 10 wt. % photoinitiator, up to 10 wt. % non-curable absorbermaterial, and up to 10 wt. % one or more additional components. Theforegoing weight percents are based on the total weight of the ink.Additionally, as understood by one of ordinary skill in the art, thetotal amount of the oligomeric curable material, monomeric curablematerial, photoinitiator, non-curable absorber material, and one or moreadditional components is equal to 100 wt. %. Moreover, thephotoinitiator is operable to initiate curing of the oligomeric curablematerial and/or the monomeric curable material when the photoinitiatoris exposed to incident curing radiation having a peak wavelength λ. Thatis, the photoinitiator is a photoinitiator of curing of the oligomericcurable material and/or the monomeric curable material. Further, the inkhas a penetration depth (D_(p)) and a critical energy (E_(c)) at thewavelength λ. The ink also has a print through depth (D_(PT)) at thewavelength λ of less than or equal to 2×D_(p).

As understood by one of ordinary skill in the art, D_(PT) refers to thetotal cure depth minus layer thickness, where the “total cure depth”refers to the depth at which any curing or polymerization of the inkoccurs in response to the incident curing radiation. “Layer thickness”refers to the thickness of the region in which “full” curing orpolymerization or curing of the ink occurs in response to the incidentcuring radiation. Such “full” curing refers to the maximum curingprovided by the incident radiation. For example, in some cases, “full”curing corresponds to 80-100% curing, 80-95% curing, 80-90% curing,85-100% curing, 85-99% curing, 85-95% curing, 90-100% curing, 90-99%curing, or 90-95% curing, where the percentage is based on the totalnumber of available curable moieties.

In some cases, an ink described herein has a D_(PT) at the wavelength λof less than or equal to 1.5×D_(p), less than or equal to 1.3×D_(p),less than or equal to 1.2×D_(p), or less than or equal to 1.1×D_(p). Insome instances, the D_(PT) at the wavelength λ is between 0.8× and2×D_(p), between 0.8× and 1.5×D_(p), between 0.9× and 2×D_(p), between0.9× and 1.8×D_(p), between 0.9× and 1.5×D_(p), between 0.9× and1.3×D_(p), between 1× and 2×D_(p), between 1× and 1.7×D_(p), between0.1× and 1.5×D_(p), between 1.1× and 2×D_(p), between 1.1× and1.5×D_(p), between 1.2× and 2×D_(p), between 1.2× and 1.8×D_(p), between1.3× and 2×D_(p), between 1.3× and 1.7×D_(p), or between 1.5× and2×D_(p).

Not intending to be bound by theory, it is believed that an ink havingsuch a D_(PT) value provides improved consistency, accuracy, andresolution when used as a build material in an additive manufacturingprocess, including an additive manufacturing process described herein.An ink described herein is further believed to reduce waste of buildmaterial and/or reduce or eliminate the occurrence of an undesired“build up” or “gummy” residue or layer on a surface of an additivemanufacturing system following completion of an additive manufacturingprocess. Such residues can be especially undesired on so-called “downsurfaces” of 3D printing systems.

Moreover, in some cases, an ink described herein has a D_(p) value andan E_(c) value at the wavelength λ that correspond to a D_(PT) valuedescribed above. For instance, in some cases, an ink described hereinhas a ratio of D_(p) to E_(c), in units of (μm cm²)/mJ, of at least 10or at least 15. In some embodiments, the ratio of D_(p) to E_(c), inunits of (μm cm²)/mJ, is between 10 and 50, between 10 and 25, between10 and 15, between 10 and 13, between 15 and 50, between 15 and 30,between 15 and 25, or between 19 and 25. Such D_(p)/E_(c) values canprovide D_(PT) values corresponding to those above.

In one “regime,” for instance, the D_(p) of the ink is 60-100 μm, andthe E_(c) of the ink is 2-4 mJ/cm². In other exemplary embodiments, theD_(p) of the ink is 101-150 μm, and the E_(c) of the ink is 4-20 mJ/cm².In still other cases, the D_(p) of the ink is 151-200 μm, and the E_(c)of the ink is 8-15 mJ/cm².

Further, in some embodiments, an ink described herein has an E_(c) valuethat permits energy-efficient and/or rapid additive manufacturing usingthe ink as a build material. For example, in some cases, an inkdescribed herein has an E_(c) of no greater than 60 mJ/cm², no greaterthan 50 mJ/cm², no greater than 40 mJ/cm², no greater than 20 mJ/cm², orno greater than 10 mJ/cm². In some instances, an ink described hereinhas an E_(c) of 1-30 mJ/cm², 1-20 mJ/cm², 1-15 mJ/cm², 1-10 mJ/cm², 2-25mJ/cm², 2-20 mJ/cm², 2-15 mJ/cm², 2-10 mJ/cm², 2-15 mJ/cm².

Similarly, an ink described herein, in some cases, has a D_(p) valuethat permits high resolution and/or rapid additive manufacturing usingthe ink. In some cases, for instance, an ink described herein has aD_(p) of 50-200 μm, 60-150 μm, 70-150 μm, or 70-100 μm.

Additionally, it is to be understood that the parameters or propertiesD_(p), E_(c), and D_(PT) are structural parameters or properties of anink described herein. A discussion of the “structural” or“compositional” nature of these values can be found, for instance, inChapter 4 of Paul F. Jacobs, Rapid Prototyping & Manufacturing:Fundamentals of Stereolithography (Society of Manufacturing Engineers,McGraw-Hill, 1992) (first edition) (hereinafter referred to as“Jacobs”). As understood by one of ordinary skill in the art, the valueD_(p) is the penetration depth of the ink, defined as that depth of theink which results in a reduction of the irradiance to a level equal to1/e of the surface irradiance, where e is the base of natural logarithms(equal to 2.7182818 . . . ). E_(c) is the critical energy, which is theenergy needed to obtain the gel point of an ink, as described on page 86of Jacobs. Moreover, as further described by Jacobs (pages 86-89), themetric E_(c) is equal to the intercept of a working curve correspondingto a semilog plot of cure depth on the ordinate and the logarithm ofmaximum radiation exposure on the abscissa. E_(c) is assigned to theintercept, at which the cure depth is zero.

It is further to be understood that the amounts of photoinitiator and/ornon-curable absorber material included in an ink described herein can beselected to obtain a desired D_(p), E_(c), and/or D_(PT) value, incombination with other components of the ink. However, it is to beunderstood that, in some instances, the other components of the ink,such as the oligomeric and monomeric curable materials, can vary in typeand/or in quantity without substantially changing the desired D_(p),E_(c), and/or D_(PT) values obtained by the particular combination ofphotoinitiator and/or non-curable absorber material. For instance, insome cases, changes in the type and/or quantity of the oligomeric andmonomeric curable materials (within the scope of the presently disclosedtypes and quantities) affect the D_(p), E_(c), and/or D_(PT) values ofan ink by 5% or less, 4% or less, 3% or less, 2% or less, or 1% or less.More particularly, such minimal changes in the D_(p), E_(c), and/orD_(PT) values can be obtained when the components of the ink other thanthe photoinitiator and non-curable absorber material (such as theoligomeric curable material and/or monomeric curable material) do notabsorb (or refract or reflect) or only minimally absorb (or refract orreflect) light of the wavelength λ. Alternatively, such minimal changesin the D_(p), E_(c), and/or D_(PT) values can also be obtained when thecomponents of the ink other than the photoinitiator and non-curableabsorber material (such as the oligomeric curable material and/ormonomeric curable material) absorb (or refract or reflect) light of thewavelength λ to approximately the same degree, no matter which precisespecies or amounts of components are selected (within the confines ofthe presently disclosed options for species and amounts). In otherwords, in the context of compositions and methods described herein, thecomponents of inks described herein, other than the photoinitiator andnon-curable absorber material, can essentially be (and generally are)optical “spectator” species at the wavelength λ, such that these“spectator” species do not substantially affect the D_(p), E_(c), and/orD_(PT) values of the overall ink. Thus, as described in more detailbelow, the oligomeric and monomeric curable materials can, in someinstances, be varied as desired from ink to ink (in terms of precisespecies and/or quantity) such that the precise species and/or quantityused from ink to ink have similar optical absorption profiles and/orrefractive indices.

A “non-curable absorber material,” for reference purposes herein, is amaterial or chemical species that is not curable or substantiallycurable by the curing radiation described herein and that absorbs atleast a portion of the curing radiation, without causing substantialcuring of other components of the ink. Thus, a “non-curable” absorbermaterial can also be referred to as a “non-curing” or “non-reactive”absorber material. Moreover, a non-curable or non-curing absorbermaterial described herein that is not “substantially” curable or thatdoes not cause “substantial” curing is understood to convert (or use)less than 5%, less than 1%, less than 0.5%, or less than 0.1% ofabsorbed curing radiation photons into (or in) a curing event. Forexample, a non-curable (or non-curing) absorber material describedherein, in some embodiments, can convert less than 2%, less than 1%,less than 0.5%, or less than 0.1% of absorbed photons into afree-radical species that can initiate or participate in (meth)acrylatepolymerization or another curing process.

It is further to be understood that a non-curable or non-curing absorbermaterial described herein can be a polymerization “spectator” (i.e.,non-polymerizing or non-polymerization-initiating) species thatnevertheless “competes” with a photoinitiator of the ink for absorptionof photons of incident curing radiation. Thus, in some cases, anon-curable absorber material and a photoinitiator of an ink describedherein have substantially overlapping photon absorption profiles,particularly in a region of the electromagnetic spectrum correspondingto or including the peak wavelength λ described above. In someinstances, for example, both the non-curable absorber material and thephotoinitiator have an absorption peak within 30 nm, within 20 nm,within 15 nm, within 10 nm, or within 5 nm of the wavelength λ.

However, it is to be understood that a non-curable absorber material anda photoinitiator of an ink described herein need not have the sameabsorbance, optical density, extenuation coefficient, and/or molarextinction coefficient at the wavelength λ or at any other specificwavelength. Instead, the non-curable absorber material and thephotoinitiator can have differing absorbances, optical densities,extenuation coefficients, and/or molar extinction coefficients at thewavelength λ, as well as at other wavelengths.

In addition, in some cases, the amount of photoinitiator and the amountof non-curable absorber material included in an ink described herein areselected based on similarities or differences between the absorbances,optical densities, extenuation coefficients, and/or molar extinctioncoefficients of the species, including at the wavelength λ. Forinstance, in some cases, the amounts of the photoinitiator and thenon-curable absorber material are selected to provide a desired ratio oftotal absorbance of each species at the wavelength λ, and/or to providea desired D_(PT), D_(p), or D_(p)/E_(c) value described hereinabove. Insome such embodiments, the total absorbance of the non-curable absorbermaterial at the wavelength λ is about 0.1 to 10 times, about 0.2 to 5times, or about 0.5 to 2 times the total absorbance of thephotoinitiator at the wavelength λ, where the “total absorbance” of eachspecies at the wavelength λ is understood to refer to the amount (inmoles) of the species times the molar extinction coefficient of thespecies at the wavelength λ.

It should further be noted that the wavelength λ can be any wavelengthnot inconsistent with the objectives of the present disclosure. Forexample, in some cases, X is a wavelength in the ultraviolet (UV) orvisible region of the electromagnetic spectrum. In some cases, the peakwavelength λ is in the infrared (IR) region of the electromagneticspectrum. In some embodiments, the wavelength λ is between 250 nm and400 nm or between 300 nm and 385 nm. In other cases, the wavelength λ isbetween 600 nm and 800 nm or between 900 nm and 1.3 μm. However, theprecise wavelength λ is not particularly limited.

Any non-curable absorber material not inconsistent with the objectivesof the present disclosure may be used in an ink described herein. Forexample, in some embodiments, a non-curable absorber material comprisesa polycyclic aromatic compound such as pyrene. A non-curable absorbermaterial may also be a “dye” that has an absorption profile consistentwith the description above. Such a “dye” may, more particularly, be ahydrophobic or oil-soluble dye. For instance, in some cases, thenon-curable absorber material is a yellow dye, such as an oil-solubleyellow dye. Other yellow dyes could also be used. In other instances, anon-curable absorber material comprises a blue or green dye, such as aKEYPLAST dye commercially available from Keystone, Inc. Additionally, insome embodiments, which may not be preferred, a non-curable absorbermaterial may have a broader absorption profile rather than a narrowerabsorption profile. For example, in some such cases, the non-curableabsorber material comprises a black dye. Carbon black or another carbonallotrope may also be used as a non-curable absorber material, in someembodiments.

As described above, the non-curable absorber material component can bepresent in an ink described herein in an amount up to 10 wt. %, based onthe total weight of the ink. For example, in some instances, an inkcomprises up to 7 wt. %, up to 5 wt. %, up to 3 wt. %, up to 2 wt. %, orup to 1 wt. % non-curable absorber material. In some embodiments, an inkcomprises 0.01-10 wt. %, 0.01-5 wt. %, 0.01-3 wt. %, 0.01-2 wt. %,0.01-1 wt. %, 0.05-10 wt. %, 0.05-5 wt. %, 0.05-3 wt. %, 0.05-1 wt. %,0.1-10 wt. %, 0.1-7 wt. %, 0.1-5 wt. %, 0.1-3 wt. %, 0.1-1 wt. %, 0.5-10wt. %, 0.5-7 wt. %, 0.5-5 wt. %, 0.5-2 wt. %, 0.5-1 wt. %, 1-10 wt. %,1-7 wt. %, 1-5 wt. %, or 1-3 wt. % non-curable absorber material, basedon the total weight of the ink. In some especially preferredembodiments, the amount of non-curable absorber material is no more thanabout 1 wt. %. For example, in some preferred embodiments, an inkdescribed herein comprises 0.0001-1 wt. %, 0.0001-0.5 wt. %, 0.0001-0.1wt. %, 0.001-1 wt. %, 0.001-0.5 wt. %, 0.001-0.1 wt. %, 0.001-0.05 wt.%, 0.01-1 wt. %, 0.01-0.5 wt. %, 0.01-0.1 wt. %, or 0.01-0.05 wt. %non-curable absorber material, based on the total weight of the ink. Theuse of a relatively small amount of non-curable absorber material, suchas one of the immediately preceding amounts, can be especiallyadvantageous for maintaining or achieving desired mechanical propertiesof an article formed from a given ink in a given instance, since the“inert” non-curable absorber material can play the role of anon-reactive “filler” as well as being an optically relevant materialduring curing.

Inks described herein also comprise one or more photoinitiators. Anyphotoinitiator not inconsistent with the objectives of the presentdisclosure may be used in an ink described herein. In some embodiments,for example, the photoinitiator comprises an alpha-cleavage type(unimolecular decomposition process) photoinitiator or a hydrogenabstraction photosensitizer-tertiary amine synergist, operable to absorblight between about 250 nm and about 400 nm or between about 300 nm andabout 385 nm, to yield free radical(s). Examples of alpha cleavagephotoinitiators are Irgacure 184 (CAS 947-19-3), Irgacure 369 (CAS119313-12-1), and Irgacure 819 (CAS 162881-26-7). An example of aphotosensitizer-amine combination is Darocur BP (CAS 119-61-9) withdiethylaminoethylmethacrylate.

In addition, in some instances, photoinitiators comprise benzoins,including benzoin, benzoin ethers, such as benzoin methyl ether, benzoinethyl ether and benzoin isopropyl ether, benzoin phenyl ether andbenzoin acetate, acetophenones, including acetophenone,2,2-dimethoxyacetophenone and 1,1-dichloroacetophenone, benzil, benzilketals, such as benzil dimethyl ketal and benzil diethyl ketal,anthraquinones, including 2-methylanthraquinone, 2-ethylanthraquinone,2-tert-butylanthraquinone, 1-chloroanthraquinone and2-amylanthraquinone, triphenylphosphine, benzoylphosphine oxides, suchas 2,4,6-trimethylbenzoyldiphenylphosphine oxide (Lucirin TPO),benzophenones, such as benzophenone and4,4′-bis(N,N′-dimethylamino)benzophenone, thioxanthones and xanthones,acridine derivatives, phenazine derivatives, quinoxaline derivatives or1-phenyl-1,2-propanedione, 2-O-benzoyl oxime, 1-aminophenyl ketones or1-hydroxyphenyl ketones, such as 1-hydroxycyclohexyl phenyl ketone,phenyl 1-hydroxyisopropyl ketone and 4-isopropylphenyl1-hydroxyisopropyl ketone.

Photoinitiators can also comprise photoinitiators operable for use witha HeCd laser radiation source, including acetophenones,2,2-dialkoxybenzophenones and 1-hydroxyphenyl ketones, such as1-hydroxycyclohexyl phenyl ketone or 2-hydroxyisopropyl phenyl ketone(=2-hydroxy-2,2-dimethylacetophenone). Additionally, in some cases,photoinitiators comprise photoinitiators operable for use with an Arlaser radiation source including benzil ketals, such as benzil dimethylketal. In some embodiments, a suitable photoinitiator comprises anα-hydroxyphenyl ketone, benzil dimethyl ketal or2,4,6-trimethylbenzoyldiphenylphosphine oxide or a mixture thereof.

Another class of photoinitiators that may be included in an inkdescribed herein comprises ionic dye-counter ion compounds capable ofabsorbing actinic radiation and generating free radicals forpolymerization initiation. Some ionic dye-counter ion compounds andtheir mode of operation are disclosed in EP-A-0 223 587 and U.S. Pat.Nos. 4,751,102; 4,772,530; and 4,772,541. A photoinitiator describedherein may also be a cationic photoinitiator such as a triphenylsulphonium photoinitiator.

A photoinitiator can be present in an ink described herein in any amountnot inconsistent with the objectives of the present disclosure. In someembodiments, a photoinitiator is present in an ink in an amount of up toabout 10 wt. %, up to about 8 wt. %, up to about 7 wt. %, up to about 5wt. %, up to about 3 wt. %, or up to about 2 wt. %, based on the totalweight of the ink. In some cases, a photoinitiator is present in anamount of about 0.1-10 wt. %, 0.1-5 wt. %, 0.1-3 wt. %, 0.1-2 wt. %,0.5-5 wt. %, 0.5-3 wt. %, 0.5-2 wt. %, 1-10 wt. %, 1-8 wt. %, 1-5 wt. %,1-4 wt. %, or 1-3 wt. %. In some especially preferred embodiments, anink described herein comprises a photoinitiator in an amount of up toabout 5 wt. %. For example, in some instances, the photoinitiatorcomponent is present in the ink in an amount of 0.1-5 wt. % or 0.5-5 wt.% or, even more preferably, 1-5 wt. %, 2-5 wt. %, or 2-4 wt. %, based onthe total weight of the ink.

It is further to be understood that the amounts (weight percents)described in the immediately preceding paragraph refer tophotoinitiators that are non-oligomeric and non-polymeric. That is, theamounts described above refer to “monomeric” or “molecular”photoinitiators, which may, for instance, have a molecular weight ofless than 400. However, it is also to be understood that oligomeric orpolymeric photoinitiators may be used in inks and methods describedherein. But in such an instance (when an oligomeric or polymericphotoinitiator is used), then the amounts (weight percents) above are tobe calculated without taking into account the weight of the oligomericor polymeric portion or moiety of the oligomeric or polymericphotoinitiator. In other words, to determine the overall amount (weightpercent) of the oligomeric or polymeric photoinitiator that is presentin the ink, the calculation (specifically, the numerator) should bebased on only the molecular weight of the photoactive moiety of thephotoinitiator, not on the molecular weight(s) of the remaining moietiesor repeating units of the oligomeric or polymeric photoinitiator (forpurposes of the present disclosure).

Moreover, as described above, the amount of photoinitiator and theamount of non-curable absorber material can be selected with referenceto each other. For example, in some cases, an ink described hereinincludes up to 5 wt. % photoinitiator and up to 1 wt. % non-curableabsorber material. In other instances, an ink described herein comprisesup to 4 wt. % photoinitiator and up to 0.5 wt. % non-curable absorbermaterial, or up to 5 wt. % photoinitiator and up to 0.05 wt. %non-curable absorber material. In some especially preferred embodiments,an ink described herein comprises at least 1 wt. % photoinitiator, incombination with an amount of non-curable absorber material describedherein, such as an amount of up to 0.5 wt. % non-curable absorbermaterial. As described further herein, compositions including too littlephotoinitiator (especially compared to the amount of non-curableabsorber material) can be insufficiently responsive to curing radiationwithin the distance D_(p), with the result that insufficientpolymerization takes place within the spatial region defined by D_(p).In some cases, a preferred ratio (by weight) of photoinitiator tonon-curable absorber material is 1 or more, 5 or more, or 10 or more. Insome embodiments, a preferred ratio (by weight) of photoinitiator tonon-curable absorber material is 1-200, 1-100, 5-100, 10-200, 10-150,10-100, 25-200, 25-100, 50-200, 50-150, or 50-100 (where the weight ofphotoinitiator is the numerator, and the weight of non-curable absorbermaterial is the denominator). Such ratios can, in some cases, provide adesired curing effect (e.g., achieving a desired D_(p), E_(c), orD_(p)/E_(c) ratio) while minimizing the amount of otherwisenon-functional or non-curing “filler” material, with respect toformation of a cured polymer network.

In addition, as described above, the relative amounts of photoinitiatorand non-curable absorber material can be based, at least in part, on thetotal (optical) absorbance of each of the photoinitiator and thenon-curable absorber material at the wavelength λ (as opposed to beingbased on only weight percent or mass). For example, if a non-curableabsorber material absorbs relatively weakly at the wavelength λ, then arelatively large amount (molar or weight percent) of non-curableabsorber material may be needed to achieve a desired “photoncompetition” with the photoinitiator, as compared to the situation whenthe non-curable absorber material absorbs relatively strongly at thewavelength λ (in which case a relatively small amount (molar or weightpercent) of non-curable absorber material may be needed to achieve thesame desired “photon competition”). Therefore, in some embodiments, aratio of photoinitiator to non-curable absorber material describedherein (such as a weight-based ratio described above) is used when thephotoinitiator and the non-curable absorber material have absorption (oroptical density) values at the wavelength λ that are within a factor of2 of one another. Moreover, in some cases, a ratio described in thepreceding paragraph (such as a ratio of photoinitiator to non-curableabsorber material within the range of 10-100) is a total opticalabsorbance ratio at the wavelength λ, rather than a weight-based ratio.

Turning now to other specific components of inks described herein, inksdescribed herein may comprise one or more oligomeric curable materialsand/or one or more monomeric curable materials. A curable material, forreference purposes herein, comprises a chemical species that includesone or more curable or polymerizable moieties. A “polymerizable moiety,”for reference purposes herein, comprises a moiety that can bepolymerized or cured to provide a printed 3D article or object. Suchpolymerizing or curing can be carried out in any manner not inconsistentwith the objectives of the present disclosure. In some embodiments, forexample, polymerizing or curing comprises irradiating a polymerizable orcurable material with electromagnetic radiation having sufficient energyto initiate a polymerization or cross-linking reaction. For instance, insome cases, ultraviolet (UV) radiation can be used. Thus, in someinstances, a polymerizable moiety comprises a photo-polymerizable orphoto-curable moiety, such as a UV-polymerizable moiety. In someembodiments, a curable material described herein is photo-polymerizableor photo-curable at wavelengths ranging from about 300 nm to about 400nm or from about 320 nm to about 380 nm. Alternatively, in otherinstances, a curable material is photo-polymerizable at visiblewavelengths of the electromagnetic spectrum. Additionally, the curingradiation generally includes the curing radiation having a peakwavelength of λ, as described above.

Moreover, a polymerization reaction, in some cases, comprises a freeradical polymerization reaction, such as that between points ofunsaturation, including points of ethyleneic unsaturation. Otherpolymerization reactions may also be used. As understood by one ofordinary skill in the art, a polymerization reaction used to polymerizeor cure a curable material described herein can comprise a reaction of aplurality of “monomers” or chemical species having one or morefunctional groups or moieties that can react with one another to formone or more covalent bonds.

One non-limiting example of a polymerizable moiety of a curable materialdescribed herein is an ethyleneically unsaturated moiety, such as avinyl moiety, allyl moiety, or (meth)acrylate moiety, where the term“(meth)acrylate” includes acrylate or methacrylate or a mixture orcombination thereof.

“Oligomeric” species, which are contained in the oligomeric curablematerial described herein, are themselves polymers or oligomers and havea relatively high molecular weight or a relatively high viscosity. Thesespecies are also capable of undergoing additional polymerization, suchas through one or more points of unsaturation described herein. Apopulation of oligomeric species in the oligomeric curable materialdescribed herein can have varying molecular structures and/or formulasthroughout the population (such as may be exhibited, for example, by aspecified mass of a urethane acrylate having a non-unity molecularweight distribution, or by a specified mass of an ethoxylatedpolyethylene glycol having a distribution of ethylene glycol unitsand/or a distribution of ethoxy units within the population). The weightaverage molecular weight of an oligomeric curable material describedherein can generally be in the range from about 400 to 10,000, fromabout 600 to 10,000, from about 500 to 7,000, or from about 500 to5,000.

In contrast to an “oligomeric” species, “monomeric” species, which arecontained in the monomeric curable material described herein, are notthemselves a polymer or oligomer, and have a relatively low molecularweight or a relatively low viscosity. “Monomeric” species contained inthe monomeric curable material can have a consistent or well-definedmolecular structure and/or formula throughout the population (such asmay be exhibited, for instance, by a specified mass of ethoxylated (4)bisphenol A diacrylate or a specific mass of the above-described curablemonomer). Additionally, in some embodiments, a monomeric curablematerial as described herein has a viscosity of 500 centipoise (cP) orless at 25° C., when measured according to ASTM D2983, while an“oligomeric” curable material has a viscosity of 1000 cP or more at 25°C., when measured according to ASTM D2983.

One non-limiting example of a polymerizable moiety of the oligomericcurable material or the monomeric curable material described herein isan ethylenically unsaturated moiety, such as a vinyl moiety, allylmoiety, or (meth)acrylate moiety, where the term “(meth)acrylate”includes acrylate or methacrylate or a mixture or combination thereof.

Additionally, the oligomeric curable material and the monomeric curablematerial described herein can comprise a monofunctional, difunctional,trifunctional, tetrafunctional, pentafunctional, or higher functionalcurable species. A “monofunctional” curable species, for referencepurposes herein, comprises a chemical species that includes one curableor polymerizable moiety. Similarly, a “difunctional” curable speciescomprises a chemical species that includes two curable or polymerizablemoieties; a “trifunctional” curable species comprises a chemical speciesthat includes three curable or polymerizable moieties; a“tetrafunctional” curable species comprises a chemical species thatincludes four curable or polymerizable moieties; and a “pentafunctional”curable species comprises a chemical species that includes five curableor polymerizable moieties. Thus, in some embodiments, a monofunctionalcurable material of an ink described herein comprises amono(meth)acrylate, a difunctional curable material of an ink describedherein comprises a di(meth)acrylate, a trifunctional curable material ofan ink described herein comprises a tri(meth)acrylate, a tetrafunctionalcurable material of an ink described herein comprises atetra(meth)acrylate, and a pentafunctional curable material of an inkdescribed herein comprises a penta(meth)acrylate. Other monofunctional,difunctional, trifunctional, tetrafunctional, and pentafunctionalcurable materials may also be used.

Moreover, a monofunctional, difunctional, trifunctional,tetrafunctional, and pentafunctional curable material, in some cases,can comprise a relatively low molecular weight species, i.e., amonomeric species, or a relatively high molecular weight species, i.e.,an oligomeric species.

In general, any oligomeric curable material or combination of oligomericcurable materials not inconsistent with the objectives of the presentdisclosure may be used in an ink described herein. For example, in somecases, oligomeric curable materials suitable for use in inks describedherein have similar wavelength absorption profiles and/or refractiveindices, including absorption profiles and/or refractive indicesdescribed hereinabove with reference to the wavelength λ or wavelengthsnear (e.g., within 30 nm of) the wavelength λ. In some instances, anoligomeric curable material described herein has a photon absorptionprofile that is outside of, or does not include, curing radiation havingthe peak wavelength λ.

In some cases, an oligomeric curable material comprises a polyester(meth)acrylate oligomer, a urethane (meth)acrylate oligomer, or anepoxy(meth)acrylate oligomer. Further, in some embodiments, anoligomeric curable material described herein comprises an aliphaticpolyester urethane acrylate oligomer and/or an acrylate amine oligomericresin, such as EBECRYL 7100. In some cases, an oligomeric curablematerial described herein comprises a polypropylene glycolmono(meth)acrylate or polyethylene glycol mono(meth)acrylate. In someembodiments, an oligomeric curable material comprises a monofunctionalaliphatic urethane (meth)acrylate. Moreover, in some cases, anoligomeric curable material comprises a diacrylate and/or dimethacrylateester of an aliphatic, cycloaliphatic or aromatic diol, includingpolyethylene glycol, ethoxylated or propoxylated neopentyl glycol,ethoxylated or propoxylated bisphenol A, ethoxylated or propoxylatedbisphenol F, ethoxylated or propoxylated bisphenol S, ethoxylated orpropoxylated 1,1,1-trimethylolpropanetri(meth)acrylate, or ethoxylatedor propoxylated glycerol tri(meth)acrylate. An oligomeric material mayalso comprise a cycloaliphatic epoxy.

Some non-limiting examples of commercially available oligomeric curablematerials useful in some embodiments described herein include thefollowing: alkoxylated tetrahydrofurfuryl acrylate, commerciallyavailable from SARTOMER under the trade name SR 611; monofunctionalurethane acrylate, commercially available from RAHN USA under the tradename GENOMER 1122; an aliphatic urethane diacrylate, commerciallyavailable from ALLNEX under the trade name EBECRYL 8402; amultifunctional acrylate oligomer, commercially available from DYMAXCorporation under the trade name BR-952; and aliphatic polyetherurethane acrylate, commercially available from DYMAX Corporation underthe trade name BR-371 S. Other commercially available oligomeric curablematerials may also be used.

Urethane (meth)acrylates suitable for use in inks described herein, insome cases, can be prepared in a known manner, typically by reacting ahydroxyl-terminated urethane with acrylic acid or methacrylic acid togive the corresponding urethane (meth)acrylate, or by reacting anisocyanate-terminated prepolymer with hydroxyalkyl acrylates ormethacrylates to give the urethane (meth)acrylate. Suitable processesare disclosed, inter alia, in EP-A 114 982 and EP-A 133 908. The weightaverage molecular weight of such (meth)acrylate oligomers, in somecases, can be from about 400 to 10,000 or from about 500 to 7,000.Urethane (meth)acrylates are also commercially available from SARTOMERunder the product names CN980, CN981, CN975 and CN2901, or from BOMARSpecialties Co. under the product name BR-741. In some embodimentsdescribed herein, a urethane (meth)acrylate oligomer has a viscosityranging from about 140,000 centipoise (cP) to about 160,000 cP at about50° C. or from about 125,000 cP to about 175,000 cP at about 50° C. whenmeasured in a manner consistent with ASTM D2983. In some cases, aurethane (meth)acrylate oligomer has a viscosity ranging from about100,000 cP to about 200,000 cP at about 50° C. or from about 10,000 cPto about 300,000 cP at about 50° C. when measured in a manner consistentwith ASTM D2983.

The oligomeric curable material can be present in an ink describedherein in any amount not inconsistent with the objectives of the presentdisclosure. In some cases, the oligomeric curable material, in total, ispresent in the ink in an amount up to about 80 wt. %, up to 70 wt. %, upto about 60 wt. %, up to about 50 wt. %, up to about 40 wt. %, up toabout 30 wt. %, or up to about 20 wt. %, based on the total weight ofthe ink. In some instances, an ink described herein comprises about10-80 wt. % or 10-70 wt. % oligomeric curable material, based on thetotal weight of the ink. In some embodiments, an ink comprises about10-60 wt. %, 10-50 wt. %, 10-40 wt. %, 10-30 wt. %, 10-20 wt. %, 15-40wt. %, 15-30 wt. %, 20-60 wt. %, 20-50 wt. %, 20-40 wt. %, 30-60 wt. %,30-50 wt. %, or 40-60 wt. % oligomeric curable material, based on thetotal weight of the ink. In some especially preferred embodiments, theamount of oligomeric curable material is no greater than about 60 wt. %,based on the total weight of the ink. In some cases, for example, an inkdescribed herein comprises 10-60 wt. %, 10-55 wt. %, 15-60 wt. %, 15-55wt. %, 15-50 wt. %, 20-60 wt. %, 20-55 wt. %, 20-50 wt. %, 25-60 wt. %,25-55 wt. %, 25-50 wt. %, 30-60 wt. %, 30-55 wt. %, 30-50 wt. %, 35-60wt. %, 35-55 wt. %, 40-60 wt. %, 40-55 wt. %, 40-50 wt. %, 45-60 wt. %,45-55 wt. %, or 50-60 wt. % oligomeric curable material, based on thetotal weight of the ink.

Moreover, when the amount of oligomeric curable material in an inkdescribed herein is greater than 60 wt. %, it is be understood thatrelatively low molecular weight oligomers are generally used andgenerally preferred. For example, if an ink described herein comprises65-80 wt. % oligomeric curable material, then the average molecularweight (e.g., the weight average molecular weight) of the oligomericcurable material may be less than 1000, as opposed to being greater than1000. Alternatively, in other instances when the amount of oligomericcurable material in an ink described herein is greater than 60 wt. %, anoligomeric curable material having a weight average molecular weightabove 1000 can be used, provided that the ink is used at an elevatedtemperature during the additive manufacturing process, such that theviscosity of the ink is similar to the viscosity of other inks describedabove in this paragraph.

In addition, any monomeric curable material or combination of monomericcurable materials not inconsistent with the objectives of the presentdisclosure may be used as the monomeric curable material component. Forexample, in some cases, monomeric curable materials suitable for use ininks described herein have similar wavelength absorption profiles and/orrefractive indices, including absorption profiles and/or refractiveindices described hereinabove with reference to the wavelength λ orwavelengths near (e.g., within 30 nm of) the wavelength λ. In someinstances, a monomeric curable material described herein has a photonabsorption profile that is outside of, or does not include, curingradiation having the peak wavelength λ.

In some cases, the monomeric curable material of an ink described hereincomprises one or more species of (meth)acrylates, such as one or moremonofunctional, difunctional, trifunctional, tetrafunctional(meth)acrylates, and/or pentafunctional (meth)acrylates. In someembodiments, for instance, a monomeric curable material comprises methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl(meth)acrylate, isobutyl (meth)acrylate, n-hexyl (meth)acrylate,2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, n-decyl(meth)acrylate, n-dodecyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate,2- or 3-hydroxypropyl (meth)acrylate, 2-methoxyethyl (meth)acrylate,2-ethoxyethyl (meth)acrylate, 2- or 3-ethoxypropyl (meth)acrylate,tetrahydrofurfuryl methacrylate, isobornyl (meth)acrylate,2-(2-ethoxyethoxy)ethyl acrylate, cyclohexyl methacrylate,2-phenoxyethyl acrylate, glycidyl acrylate, isodecyl acrylate,2-phenoxyethyl (meth)acrylate, lauryl methacrylate, or a combinationthereof. In some embodiments, a monomeric curable material comprises oneor more of allyl acrylate, allyl methacrylate, triethylene glycoldi(meth)acrylate, tricyclodecane dimethanol diacrylate, and cyclohexanedimethanol diacrylate. Additionally, in some cases, a monomeric curablematerial comprises diacrylate and/or dimethacrylate esters of aliphatic,cycloaliphatic or aromatic diols, including 1,3- or 1,4-butanediol,neopentyl glycol, 1,6-hexanediol, diethylene glycol, triethylene glycol,tetraethylene glycol, tripropylene glycol,1,4-dihydroxymethylcyclohexane, 2,2-bis(4-hydroxycyclohexyl)propane orbis(4-hydroxycyclohexyl)methane, hydroquinone, 4,4′-dihydroxybiphenyl,bisphenol A, bisphenol F, or bisphenol S. A monomeric curable materialdescribed herein may also comprise 1,1-trimethylolpropanetri(meth)acrylate, pentaerythritol monohydroxy tri(meth)acrylate,dipentaerythritol monohydroxy penta(meth)acrylate, and/orbis(trimethylolpropane) tetra(meth)acrylate. Further, in some cases, amonomeric curable material can comprise an ethoxylated or propoxylatedspecies, such as ethoxylated or propoxylated neopentyl glycol,ethoxylated or propoxylated bisphenol A, ethoxylated or propoxylatedbisphenol F, ethoxylated or propoxylated bisphenol S, ethoxylated orpropoxylated 1,1,1-trimethylolpropanetri(meth)acrylate, or ethoxylatedor propoxylated glycerol tri(meth)acrylate. In some cases, a monomericcurable material comprises a cycloaliphatic epoxy.

Additional non-limiting examples of commercially available monomericcurable materials useful in some embodiments described herein includethe following: isobornyl acrylate (IBOA), commercially available fromSARTOMER under the trade name SR 506; isobornyl methacrylate,commercially available from SARTOMER under the trade name SR 423A;triethylene glycol diacrylate, commercially available from SARTOMERunder the trade name SR 272; triethylene glycol dimethacrylate,commercially available from SARTOMER under the trade name SR 205;tricyclodecane dimethanol diacrylate, commercially available fromSARTOMER under the trade name SR 8335; tris(2-hydroxy ethyl)isocyanuratetriacrylate, commercially available from SARTOMER under the trade nameSR 368; 2-phenoxyethyl acrylate, commercially available from SARTOMERunder the trade name SR 339; ethoxylated (3 mole) bisphenol Adiacrylate, commercially available from SARTOMER under the trade name SR349; a cyclic monofunctional acrylate, commercially available by RAHNUSA Corp. under the trade name GENOMER 1120; and dipentaerythritolpentaacrylate, commercially available from SARTOMER under the trade nameSR 399 LV. Other commercially available monomeric curable materials mayalso be used.

The monomeric curable material can be present in an ink described hereinin any amount not inconsistent with the objectives of the presentdisclosure. In some cases, the monomeric curable material, in total, ispresent in an amount up to about 80 wt. %, up to about 70 wt. %, up toabout 60 wt. %, or up to about 50 wt. %, based on the total weight ofthe ink. In some cases, an ink described herein comprises about 0-80 wt.% or 10-80 wt. % monomeric curable material, based on the total weightof the ink. In some embodiments, an ink comprises about 0-75 wt. %, 0-70wt. %, 0-60 wt. %, 0-50 wt. %, 0-40 wt. %, 0-35 wt. %, 0-30 wt. %, 0-25wt. %, 0-20 wt. %, 0-15 wt. %, 0-10 wt. %, 0-5 wt. %, 10-75 wt. %, 10-70wt. %, 10-60 wt. %, 10-50 wt. %, 10-40 wt. %, 10-35 wt. %, 10-30 wt. %,10-25 wt. %, 10-20 wt. %, 20-80 wt. %, 20-60 wt. %, or 20-40 wt. %monomeric curable material, based on the total weight of the ink.

Turning to possible additional components of inks described herein, inksdescribed herein can further comprise one or more photosensitizers. Ingeneral, such a sensitizer can be added to an ink to increase theeffectiveness of one or more photoinitiators that may also be present.In some cases, a sensitizer comprises isopropylthioxanthone (ITX) or2-chlorothioxanthone (CTX).

A sensitizer can be present in an ink in any amount not inconsistentwith the objectives of the present disclosure. In some embodiments, asensitizer is present in an amount ranging from about 0.1 wt. % to about2 wt. % or from about 0.5 wt. % to about 1 wt. %, based on the totalweight of the ink. However, in other cases, an ink described hereinexcludes a sensitizer such as described above.

Turning to another possible component of the ink described herein, inksdescribed herein can also comprise at least one colorant, which may bedifferent from the non-curable absorber material of the ink. Such acolorant of an ink described herein can be a particulate colorant, suchas a particulate pigment, or a molecular colorant, such as a moleculardye. Any such particulate or molecular colorant not inconsistent withthe objectives of the present disclosure may be used. In some cases, forinstance, the colorant of an ink comprises an inorganic pigment, such asTiO₂ and/or ZnO. In some embodiments, the colorant of an ink comprises acolorant for use in a RGB, sRGB, CMY, CMYK, L*a*b*, or Pantone®colorization scheme. Moreover, in some cases, a particulate colorantdescribed herein has an average particle size of less than about 5 μm,or less than about 1 μm. In some instances, a particulate colorantdescribed herein has an average particle size of less than about 500 nm,such as an average particle size of less than about 400 nm, less thanabout 300 nm, less than about 250 nm, less than about 200 nm, or lessthan about 150 nm. In some instances, a particulate colorant has anaverage particle size of about 50-5000 nm, about 50-1000 nm, or about50-500 nm.

A colorant can be present in an ink described herein in any amount notinconsistent with the objectives of the present disclosure. In somecases, colorant is present in the ink in an amount up to about 2 wt. %,or an amount of about 0.005-2 wt. %, 0.01-2 wt. %, 0.01-1.5 wt. %,0.01-1 wt. %, 0.01-0.5 wt. %, 0.1-2 wt. %, 0.1-1 wt. %, 0.1-0.5 wt. %,or 0.5-1.5 wt. %, based on the total weight of the ink.

Moreover, inks described herein, in some embodiments, further compriseone or more polymerization inhibitors and/or stabilizing agents. Apolymerization inhibitor can be added to an ink to provide additionalthermal stability to the composition. Any polymerization inhibitor notinconsistent with the objectives of the present disclosure may be used.Moreover, a polymerization inhibitor can retard or decrease the rate ofpolymerization, and/or prevent polymerization from occurring for someperiod of time or “induction time” until the polymerization inhibitor isconsumed. Further, in some cases, a polymerization inhibitor describedherein is an “addition type” inhibitor. An inhibitor described hereincan also be a “chain transfer type” inhibitor. In some instances, asuitable polymerization inhibitor comprises methoxyhydroquinone (MEHQ).

A stabilizing agent, in some embodiments, comprises one or moreanti-oxidants. A stabilizing agent can comprise any anti-oxidant notinconsistent with the objectives of the present disclosure. In somecases, suitable anti-oxidants include various aryl compounds, includingbutylated hydroxytoluene (BHT), which can also be used as apolymerization inhibitor in some embodiments described herein. Moregenerally, a single species may serve as both a stabilizing agent and apolymerization inhibitor. It is also possible, in some cases, to use aplurality of inhibitors and/or stabilizing agents, wherein differinginhibitors and/or stabilizers provide differing effects and/or worksynergistically.

A polymerization inhibitor and/or a stabilizing agent can be present inan ink in any amount not inconsistent with the objectives of the presentdisclosure. In some embodiments, a polymerization inhibitor is presentin an amount ranging from about 0.01 wt. % to about 2 wt. % or fromabout 0.05 wt. % to about 1 wt. %. Similarly, in some cases, astabilizing agent is present in an ink in an amount ranging from about0.1 wt. % to about 5 wt. %, from about 0.5 wt. % to about 4 wt. %, orfrom about 1 wt. % to about 3 wt. %, based on the total weight of theink.

In some embodiments, an ink described herein may contain viscositymodifying agents. Non-limiting examples of viscosity modifying agentsinclude a saturated fatty acid or a combination of saturated fattyacids, or an oil, such as a plant oil. The inks described herein maycomprise up to 5 wt. % up to 3 wt. %, up to 1 wt. %, up to 0.5 wt. %, orup to 0.1 wt. % of a viscosity modifying agent not inconsistent with theobjectives of the present disclosure.

Inks described herein can exhibit a variety of desirable properties. Forexample, an ink described herein can have any freezing point, meltingpoint, and/or other phase transition temperature not inconsistent withthe objectives of the present disclosure. In some cases, an ink hasfreezing and melting points consistent with temperatures used in some 3Dprinting systems, including 3D printing systems designed for use withphase changing inks. In some embodiments, the freezing point of an inkis greater than about 40° C. In some instances, for example, an ink hasa freezing point centered at a temperature ranging from about 45° C. toabout 55° C. or from about 50° C. to about 80° C. In some cases, an inkhas a freezing point below about 40° C. or below about 30° C.

Further, in some embodiments described herein, an ink exhibits a sharpfreezing point or other phase transition. In some cases, for instance,an ink freezes over a narrow range of temperatures, such as a range ofabout 1-10° C., about 1-8° C., or about 1-5° C. In some embodiments, anink having a sharp freezing point freezes over a temperature range ofX±2.5° C., where X is the temperature at which the freezing point iscentered (e.g., X=65° C.).

In addition, an ink described herein, in some cases, is fluid at jettingtemperatures encountered in some 3D printing systems. Moreover, in someembodiments, an ink solidifies once deposited on a surface during thefabrication of a three-dimensionally printed article or object.Alternatively, in other instances, an ink remains substantially fluidupon deposition on a surface. Solidification of an ink, in someembodiments, occurs through a phase change of the ink or a component ofthe ink. The phase change can comprise a liquid to solid phase change ora liquid to semi-solid phase change. Further, in some instances,solidification of an ink comprises an increase in viscosity of the ink,such as an increase in viscosity from a low viscosity state to a highviscosity state. Solidification of an ink can also occur due to curingof the ink.

Additionally, in some embodiments, the inks described herein, whennon-cured, have a viscosity profile consistent with the requirements andparameters of one or more 3D printing systems, such as an MJP or SLAsystem. For example, in some cases, an ink described herein has adynamic viscosity at 30° C. of 1600 centipoise (cP) or less, 1200 cP orless, or 800 cP or less. In a preferred embodiment, an ink describedherein has a dynamic viscosity of 500 cP or less at 30° C., whenmeasured according to ASTM standard D2983 (e.g., using a BrookfieldModel DV-II+ Viscometer). In some cases, an ink described herein whennon-cured exhibits a dynamic viscosity of about 200-1600 cP, about200-1200 cP, about 200-800 cP, about 200-500 cP, or about 200-400 cP at30° C., when measured according to ASTM D2983.

Inks described herein can also exhibit a variety of desirableproperties, in addition to those described hereinabove, in a curedstate. An ink in a “cured” state, as used herein, comprises an ink thatincludes a curable material or polymerizable component that has been atleast partially cured, i.e., at least partially polymerized and/orcross-linked. For instance, in some cases, a cured ink is at least about70% polymerized or cross-linked or at least about 80% polymerized orcross-linked. In some embodiments, a cured ink is at least about 85%, atleast about 90%, at least about 95%, at least about 98%, or at least 99%polymerized or cross-linked. In some instances, a cured ink is betweenabout 80% and about 99% polymerized or cross-linked.

In some cases, an ink described herein, when cured, has an elongation atbreak of about 10 to 70%, about 10 to 60%, about 15 to 50%, or about 20to 50%, when measured according to ASTM D638. Further, a cured inkdescribed herein, in some cases, can have a tensile strength of about 40to 70 MPa about 40 to 60 MPa, or about 45 to 55 MPa when measuredaccording to ASTM D638. Additionally, a cured ink described herein, insome embodiments, can have a tensile modulus of about 1800 to 2100 MPa,about 1900 to 2100 MPa, or about 1950 to 2050 MPa when measuredaccording to ASTM D638. Also, a cured ink described herein can have animpact resistance of 1 to 4 ft-lb/in (Notched), 1 to 3 ft-lb/in(Notched), or 1 to 2 ft-lb/in (Notched) when measured according to ASTMD256. Finally, in some cases, a cured ink described herein has a flexualmodulus of 2000 to 2500 MPa, 2100 to 2400 MPa, or 2100 to 2200 MPa whenmeasured according to ASTM D790.

Moreover, in some cases, an ink described herein, when cured, canexhibit a plurality of the foregoing properties. For example, in someembodiments, an ink when cured has a tensile strength of about 40-70 MPawhen measured according to ASTM D638; an impact resistance of 1 to 4ft-lb/in (Notched), when measured according to ASTM D256; and anelongation at break of about 10-70% when measured according to ASTMD638.

Inks described herein can be produced in any manner not inconsistentwith the objectives of the present disclosure. In some embodiments, forinstance, a method for the preparation of an ink described hereincomprises the steps of mixing the components of the ink, melting themixture, and filtering the molten mixture. Melting the mixture, in somecases, is carried out at a temperature of about 75° C. or in a rangefrom about 75° C. to about 85° C. In some embodiments, an ink describedherein is produced by placing all components of the ink in a reactionvessel and heating the resulting mixture to a temperature ranging fromabout 75° C. to about 85° C. with stirring. The heating and stirring arecontinued until the mixture attains a substantially homogenized moltenstate. In general, the molten mixture can be filtered while in aflowable state to remove any large undesirable particles that mayinterfere with jetting or extrusion or other printing process. Thefiltered mixture can then be cooled to ambient temperatures and storeduntil ready for use in a 3D printing system.

Inks described herein can also include, have, or exhibit any combinationof components and/or properties described hereinabove individually,provided that the combination of components and/or properties is notinconsistent with the principles and objectives of the presentinvention. For example, in some preferred embodiments, an ink describedherein comprises up to 60 wt. % oligomeric curable material; up to 80wt. % monomeric curable material; up to 5 wt. % photoinitiator; up to 1wt. % non-curable absorber material; and up to 10 wt. % one or moreadditional components, based on the total weight of the ink, wherein thetotal amount of the oligomeric curable material, monomeric curablematerial, photoinitiator, non-curable absorber material, and one or moreadditional components is equal to 100 wt. %; wherein the photoinitiatoris operable to initiate curing of the oligomeric curable material and/orthe monomeric curable material when the photoinitiator is exposed toincident curing radiation having a Gaussian distribution of wavelengthsand a peak wavelength λ; wherein the ink has a penetration depth (D_(p))and a critical energy (E_(c)) at the wavelength λ; and wherein the inkhas a print through depth (D_(PT)) at the wavelength λ of less than orequal to 2×D_(p), or a D_(p)/E_(c) value of 10-50 (μm cm²)/mJ. In otherpreferred embodiments, an ink described herein comprises 40-60 wt. %oligomeric curable material, 40-60 wt. % monomeric curable material, 1-5wt. % photoinitiator, and 0.001-0.1 wt. % non-curable absorber material,based on the total weight of the ink, wherein the total amount of theoligomeric curable material, monomeric curable material, photoinitiator,non-curable absorber material, and one or more additional components isequal to 100 wt. %; wherein the photoinitiator is operable to initiatecuring of the oligomeric curable material and/or the monomeric curablematerial when the photoinitiator is exposed to incident curing radiationhaving a Gaussian distribution of wavelengths and a peak wavelength λ;wherein the ink has a penetration depth (D_(p)) and a critical energy(E_(c)) at the wavelength λ; wherein the ink has a print through depth(D_(PT)) at the wavelength λ of less than or equal to 1.5×D_(p); andwherein E_(c) is less than 15 mJ/cm². Moreover, in still other preferredembodiments, an ink described in this paragraph further has one, atleast two, or, in especially preferred embodiments, all of the followingcharacteristics: (1) a ratio of D_(p) to E_(c), in units of (μm cm²)/mJ,of between 10 and 50; (2) a ratio of photoinitiator to non-curableabsorber material, by weight, of between 5 and 100; and (3) a D_(p) ofno greater than 200 μm or no greater than 100 μm. Inks having suchcharacteristics can be especially preferred for providing improvedaccuracy and/or precision of additive manufacturing while alsomaintaining a normal (or faster) speed of the additive manufacturingprocess, while also maintaining (or improving) normal energy efficiencyof the additive manufacturing process (in terms of energy required forcuring), and/or while also maintaining (or improving) desired mechanicalproperties of the printed articles. It is to be understood that “normal”or “maintained” characteristics as described above are relative to inksthat are comparable to inventive inks according to the presentdisclosure/preferred embodiments, but that do not fall within theinventive metrics identified above. Similarly, it is further to beunderstood that “desired mechanical properties” can vary based on agiven selection of ink components. Again, however, inks such as thepreferred inks described above can provide the advantages contemplatedin this disclosure without substantial loss of mechanical properties theinks would otherwise provide if they (the inks) fell outside of theinventive parameters described herein. For example, an ink that isformulated to have high elongation or tensile strength (e.g., throughselection of specific monomeric and/or oligomeric curable materials) canmaintain such elongation or tensile strength despite the inclusion of aphotoinitiator and non-curable absorbable material in the formulation ina manner consistent with the preferred embodiments above (e.g., anelongation or tensile strength can be achieved with preferred inksdescribed herein, wherein the elongation or tensile strength deviates byno greater than 5% from the desired elongation or tensile strengthvalue, using the desired value as the denominator for calculating thepercent deviation).

II. Methods of Forming a 3D Article

In another aspect, methods of forming or “printing” a 3D article orobject by additive manufacturing are described herein. Methods offorming a 3D article or object described herein can include forming the3D article from a plurality of layers of an ink described herein in alayer-by-layer manner. Methods of forming a 3D article by additivemanufacturing can also include forming the object in a manner other thana layer-by-layer manner. Any ink described hereinabove in Section I maybe used in a method described herein.

For example, in some cases, a method described herein comprisesproviding an ink having a penetration depth (D_(p)) and a criticalenergy (E_(c)) at a wavelength λ; and selectively curing a portion ofthe ink using incident curing radiation having a Gaussian distributionof wavelengths and a peak wavelength at the wavelength λ, wherein theink has a print through depth (D_(PT)) at the wavelength λ of less thanor equal to 2×D_(p), and/or a ratio of D_(p) to E_(c), in units of (μmcm²)/mJ, of 10-50. Moreover, in some embodiments described herein, theink is selectively cured according to preselected computer aided design(CAD) parameters, and the D_(p) corresponds to a voxel depth of the CADparameters. Moreover, in some cases, one or more layers of an inkdescribed herein has a thickness of about 10 μm to about 100 μm, about10 μm to about 80 μm, about 10 μm to about 50 μm, about 20 μm to about100 μm, about 20 μm to about 80 μm, or about 20 μm to about 40 μm. Otherthicknesses are also possible.

Additionally, it is to be understood that methods of printing a 3Darticle described herein can include, for example, MJP or SLA 3Dprinting methods. For example, in some instances, a MJP method ofprinting a 3D article comprises selectively depositing layers of an inkdescribed herein in a fluid state onto a substrate, such as a build padof a 3D printing system. In addition, in some embodiments, a methoddescribed herein further comprises supporting at least one of the layersof the ink with a support material. Any support material notinconsistent with the objectives of the present disclosure may be used.

A method described herein can also comprise curing the layers of theink, including with curing radiation described above (such as curingradiation having a peak wavelength λ). Moreover, curing can comprisepolymerizing one or more polymerizable moieties or functional groups ofone or more components of the ink. In some cases, a layer of depositedink is cured prior to the deposition of another or adjacent layer ofink. Additionally, curing one or more layers of deposited ink, in someembodiments, is carried out by exposing the one or more layers toelectromagnetic radiation, such as UV light, visible light, or infraredlight, as described above.

Further details regarding various methods, including “materialdeposition” methods (such as MJP) or “vat polymerization” methods (suchas SLA), are provided below.

A. Material Deposition Methods

In a material deposition method, one or more layers of an ink describedherein are selectively deposited onto a substrate and cured. Curing ofthe ink may occur after selective deposition of one layer, each layer,several layers, or all layers of the ink.

In some instances, an ink described herein is selectively deposited in afluid state onto a substrate, such as a build pad of a 3D printingsystem. Selective deposition may include, for example, depositing theink according to preselected CAD parameters. For example, in someembodiments, a CAD file drawing corresponding to a desired 3D article tobe printed is generated and sliced into a sufficient number ofhorizontal slices. Then, the ink is selectively deposited, layer bylayer, according to the horizontal slices of the CAD file drawing toprint the desired 3D article. A “sufficient” number of horizontal slicesis the number necessary for successful printing of the desired 3Darticle, e.g., to produce it accurately and precisely.

Further, in some embodiments, a preselected amount of ink describedherein is heated to the appropriate temperature and jetted through aprint head or a plurality of print heads of a suitable inkjet printer toform a layer on a print pad in a print chamber. In some cases, eachlayer of ink is deposited according to preselected CAD parameters. Asuitable print head to deposit the ink, in some embodiments, is apiezoelectric print head. Additional suitable print heads for thedeposition of ink and support material described herein are commerciallyavailable from a variety of ink jet printing apparatus manufacturers.For example, Xerox, Hewlett Packard, or Ricoh print heads may be used insome instances.

Additionally, in some embodiments, an ink described herein remainssubstantially fluid upon deposition. Alternatively, in other instances,the ink exhibits a phase change upon deposition and/or solidifies upondeposition. Moreover, in some cases, the temperature of the printingenvironment can be controlled so that the jetted droplets of inksolidify on contact with the receiving surface. In other embodiments,the jetted droplets of ink do not solidify on contact with the receivingsurface, remaining in a substantially fluid state. Additionally, in someinstances, after each layer is deposited, the deposited material isplanarized and cured with electromagnetic (e.g., UV, visible, orinfrared light) radiation prior to the deposition of the next layer.Optionally, several layers can be deposited before planarization andcuring, or multiple layers can be deposited and cured followed by one ormore layers being deposited and then planarized without curing.Planarization corrects the thickness of one or more layers prior tocuring the material by evening the dispensed material to remove excessmaterial and create a uniformly smooth exposed or flat up-facing surfaceon the support platform of the printer. In some embodiments,planarization is accomplished with a wiper device, such as a roller,which may be counter-rotating in one or more printing directions but notcounter-rotating in one or more other printing directions. In somecases, the wiper device comprises a roller and a wiper that removesexcess material from the roller. Further, in some instances, the wiperdevice is heated. It should be noted that the consistency of the jettedink described herein prior to curing, in some embodiments, shoulddesirably be sufficient to retain its shape and not be subject toexcessive viscous drag from the planarizer.

Moreover, a support material, when used, can be deposited in a mannerconsistent with that described hereinabove for the ink. The supportmaterial, for example, can be deposited according to the preselected CADparameters such that the support material is adjacent or continuous withone or more layers of the ink. Jetted droplets of the support material,in some embodiments, solidify or freeze on contact with the receivingsurface. In some cases, the deposited support material is also subjectedto planarization, curing, or planarization and curing. Any supportmaterial not inconsistent with the objectives of the present disclosuremay be used.

Layered deposition of the ink and support material can be repeated untilthe 3D article has been formed. In some embodiments, a method ofprinting a 3D article further comprises removing the support materialfrom the ink.

Curing of the ink may occur after selective deposition of one layer ofink, of each layer of ink, of several layers of ink, or of all layers ofthe ink necessary to print the desired 3D article. In some embodiments,a partial curing of the deposited ink is performed after selectivedeposition of one layer of ink, each layer of ink, several layers ofink, or all layers of the ink necessary to print the desired 3D article.A “partially cured” ink, for reference purposes herein, is one that canundergo further curing. For example, a partially cured ink is up toabout 30% polymerized or cross-linked or up to about 50% polymerized orcross-linked. In some embodiments, a partially cured ink is up to about60%, up to about 70%, up to about 80%, up to about 90%, or up to about95% polymerized or cross-linked.

Partial curing of the deposited ink can include irradiating the ink withan electromagnetic radiation source or photocuring the ink (includingwith curing radiation described hereinabove). Any electromagneticradiation source not inconsistent with the objectives of the presentdisclosure may be used, e.g., an electromagnetic radiation source thatemits UV, visible or infrared light. For example, in some embodiments,the electromagnetic radiation source can be one that emits light havinga wavelength from about 300 nm to about 900 nm, e.g., a Xe arc lamp.

Further, in some embodiments, a post-curing is performed after partiallycuring is performed. For example, in some cases, post-curing is carriedout after selectively depositing all layers of the ink necessary to forma desired 3D article, after partially curing all layers of the ink, orafter both of the foregoing steps have been performed. Moreover, in someembodiments, post-curing comprises photocuring, including with curingradiation described hereinabove having a peak wavelength λ. Again, anyelectromagnetic radiation source not inconsistent with the objectives ofthe present disclosure may be used for a post-curing step describedherein. For example, in some embodiments, the electromagnetic radiationsource can be a light source that has a higher energy, a lower energy,or the same energy as the electromagnetic radiation source used forpartial curing. In some cases wherein the electromagnetic radiationsource used for post-curing has a higher energy (i.e., a shorterwavelength) than that used for partial curing, a Xe arc lamp can be usedfor partial curing and a Hg lamp can be used for post-curing.

Additionally, after post-curing, in some cases, the deposited layers ofink are at least about 80% polymerized or cross-linked or at least about85% polymerized or cross-linked. In some embodiments, the depositedlayers of ink are at least about 90%, at least about 95%, at least about98%, or at least about 99% polymerized or cross-linked. In someinstances, the deposited layers of ink are about 80-100%, about 80-99%,about 80-95%, about 85-100%, about 85-99%, about 85-95%, about 90-100%,or about 90-99% polymerized or cross-linked.

B. Vat Polymerization Methods

It is also possible to form a 3D article from an ink described hereinusing a vat polymerization method, such as an SLA method. Thus, in somecases, a method of printing a 3D article described herein comprisesretaining an ink described herein in a fluid state in a container andselectively applying energy (particularly, for instance, the curingradiation having the peak wavelength λ) to the ink in the container tosolidify at least a portion of a fluid layer of the ink, thereby forminga solidified layer that defines a cross-section of the 3D article.Additionally, a method described herein can further comprise raising orlowering the solidified layer of ink to provide a new or second fluidlayer of unsolidified ink at the surface of the fluid ink in thecontainer, followed by again selectively applying energy (e.g., thecuring radiation) to the ink in the container to solidify at least aportion of the new or second fluid layer of the ink to form a secondsolidified layer that defines a second cross-section of the 3D article.Further, the first and second cross-sections of the 3D article can bebonded or adhered to one another in the z-direction (or build directioncorresponding to the direction of raising or lowering recited above) bythe application of the energy for solidifying the ink. Moreover, in someinstances, the electromagnetic radiation has an average wavelength of300-900 nm, and in other embodiments the electromagnetic radiation hasan average wavelength that is less than 300 nm. In some cases, thecuring radiation is provided by a computer controlled laser beam. Inaddition, in some cases, raising or lowering a solidified layer of inkis carried out using an elevator platform disposed in the container offluid ink. A method described herein can also comprise planarizing a newlayer of fluid ink provided by raising or lowering an elevator platform.Such planarization can be carried out, in some cases, by a wiper orroller.

It is further to be understood that the foregoing process can berepeated a desired number of times to provide the 3D article. Forexample, in some cases, this process can be repeated “n” number oftimes, wherein n can be up to about 100,000, up to about 50,000, up toabout 10,000, up to about 5000, up to about 1000, or up to about 500.Thus, in some embodiments, a method of printing a 3D article describedherein can comprise selectively applying energy (e.g., curing radiationof peak wavelength λ) to an ink in a container to solidify at least aportion of an nth fluid layer of the ink, thereby forming an nthsolidified layer that defines an nth cross-section of the 3D article,raising or lowering the nth solidified layer of ink to provide an(n+1)th layer of unsolidified ink at the surface of the fluid ink in thecontainer, selectively applying energy to the (n+1)th layer of ink inthe container to solidify at least a portion of the (n+1)th layer of theink to form an (n+1)th solidified layer that defines an (n+1)thcross-section of the 3D article, raising or lowering the (n+1)thsolidified layer of ink to provide an (n+2)th layer of unsolidified inkat the surface of the fluid ink in the container, and continuing torepeat the foregoing steps to form the 3D article. Further, it is to beunderstood that one or more steps of a method described herein, such asa step of selectively applying energy (e.g., curing radiation describedherein) to a layer of ink, can be carried out according to an image ofthe 3D article in a computer-readable format. General methods of 3Dprinting using stereolithography are further described, inter alia, inU.S. Pat. Nos. 5,904,889 and 6,558,606.

Performing a printing process described above can provide a printed 3Darticle from an ink described herein that has a high feature resolution.The “feature resolution” of an article, for reference purposes herein,can be the smallest controllable physical feature size of the article.The feature resolution of an article can be described in terms of a unitof distance such as microns (μm), or in terms of dots per inch (dpi). Asunderstood by one of ordinary skill in the art, a higher featureresolution corresponds to a higher dpi value but a lower distance valuein μm. In some cases, an article formed by depositing or solidifying anink described herein can have a feature resolution of about 500 μm orless, about 200 μm or less, about 100 μm or less, or about 50 μm orless, including at elevated temperatures. In some embodiments, anarticle has a feature resolution between about 50 μm and about 500 μm,between about 50 μm and about 200 μm, between about 50 μm and about 100μm, or between about 100 μm and about 200 μm. Correspondingly, in someinstances, an article described herein has a feature resolution of atleast about 100 dpi, at least about 200 dpi, at least about 250 dpi, atleast about 400 dpi, or at least about 500 dpi. In some cases, thefeature resolution of an article is between about 100 dpi and about 600dpi, between about 100 dpi and about 250 dpi, or between about 200 dpiand about 600 dpi.

In a vat polymerization method such as described above, the ink may bepartially cured as described in Section IIA above. For example, in someembodiments, selectively applying energy to the ink in the container tosolidify at least a portion of a fluid layer of the ink may includepartially curing at least a portion of a fluid layer of the ink. Inother embodiments, partial curing of at least a portion of a fluid layerof the ink may occur after a first layer of the ink is provided andsolidified, before or after a second layer of the ink is provided orsolidified, or before or after one, several, or all subsequent layers ofthe ink are provided or solidified.

Additionally, in some embodiments of a vat polymerization methoddescribed herein, after partial curing or after the desired 3D articleis formed, post-curing as described in Section IIA above may beperformed. The desired 3D article may be, for example, an article thatcorresponds to the design in a CAD file.

III. Printed 3D Articles

In another aspect, printed 3D articles are described herein. In someembodiments, a printed 3D article is formed from an ink describedherein. Any ink described hereinabove in Section I may be used. Forexample, in some cases, the ink comprises up to 60 wt. % oligomericcurable material; up to 80 wt. % monomeric curable material; up to 5 wt.% photoinitiator; up to 1 wt. % non-curable absorber material; and up to10 wt. % one or more additional components, based on the total weight ofthe ink, wherein the total amount of the foregoing components is equalto 100 wt. %. Additionally, the photoinitiator is operable to initiatecuring of the oligomeric curable material and/or the monomeric curablematerial when the photoinitiator is exposed to incident curing radiationhaving a peak wavelength λ. Moreover, the ink has a penetration depth(D_(p)), a critical energy (E_(c)), and a print through depth (D_(PT))at the wavelength λ of less than or equal to 2×D_(p).

Some embodiments of inks for 3D printing are also further illustrated inthe following non-limiting Examples.

Example 1 Method of Preparing Inks

Inks according to some embodiments described herein were prepared asfollows. Specifically, to prepare various inks, the components in thefollowing Tables I-VI were mixed in a reaction vessel to form specificinks, as identified in the Tables. The amounts of various components inTables I-VI refer to the wt. % of each component of the identified ink,based on the total weight of the ink. For each ink, the appropriatemixture was heated to a temperature of about 75-85° C. with stirring.The heating and stirring were continued until the mixture attained asubstantially homogenized molten state. The molten mixture was thenfiltered. Next, the filtered mixture was allowed to cool to ambienttemperature.

Example 2 Different Oligomeric and Monomeric Curable Material Amounts

Inks 1 and 2 in Table I were prepared according to the procedure inExample 1. The amounts of the oligomeric curable material and themonomeric curable material were varied while the amounts ofphotoinitiator and non-curable absorber material remained constant. Theoligomeric curable materials and the monomeric curable materials in Inks1 and 2 were the same, in terms of chemical identity, consisting of(meth)acrylates that are non-absorbing at the wavelength λ (which was405 nm). The photoinitiator was the same in each of Inks 1 and 2,consisting of Irgacure 819. The non-curable absorber material was thesame in each of Inks 1 and 2, consisting of a 1:1 mixture by weight ofOil Yellow and Blue B from Keystone. The amounts of the variouscomponents of Inks 1 and 2 in Table I are provided as weight percentages(wt. %), based on the total weight of each ink. Values of D_(p) andE_(c) are also provided in Table I for each ink. The units for thesevalues in Table I (and subsequent tables) are as follows: D_(p) (μm),E_(c) (mJ/cm²), and D_(p)/E_(c) Ratio ((μm cm²)/mJ).

TABLE I Ink Compositions. Ink 1 Ink 2 Oligomeric Curable Material 28 37Monomeric Curable Material 68.94 59.94 Photoinitiator 3 3 Non-CurableAbsorber Material 0.06 0.06 D_(p) 77.4 78.0 E_(c) 3.33 3.48 D_(P)/E_(C)Ratio 23.2 22.4

As shown in Table I, D_(p), E_(c), and the ratio D_(p)/E_(c) (and thusalso D_(PT)) remain substantially constant across differentconcentrations of the oligomeric and monomeric curable materials,indicating that the amounts of photoinitiator and non-curable absorbermaterial exert primary control over D_(p), E_(c), and D_(PT) for thesecompositions (as well as for other compositions in which the curablematerials are optical spectators, as described further hereinabove andin Example 3 below).

Example 3 Different Oligomeric and Monomeric Curable Materials

Inks 3-6 in Table II were prepared according to the procedure in Example1 (again, the components are provided in Table II as weightpercentages). In Inks 3 and 4, the concentrations of the oligomeric andmonomeric curable materials, photoinitiator, and non-curable absorbermaterial remained substantially constant, and the oligomeric curablematerial, photoinitiator, and non-curable absorber materials were thesame. However, the type or species of monomeric curable material wasdifferent for each of Inks 3 and 4. Specifically, the Inks includeddifferent species of (meth)acrylate monomers. But the monomeric curablematerials for both Inks 3 and 4 were substantially non-absorbing,optical spectator species at the wavelength λ (405 nm).

In Inks 5 and 6, the concentrations of the oligomeric and monomericcurable materials, photoinitiator, and non-curable absorber materialremained substantially constant. Additionally, the photoinitiator andnon-curable absorber materials were the same species in both Inks.However, the type or species of oligomeric curable material wasdifferent for each of Inks 5 and 6. Specifically, the Inks includeddifferent species of aliphatic urethane acrylates (Ink 6 includedtriacrylate, while Ink 5 included only diacrylate). The oligomericcurable materials for both Inks 5 and 6 were substantiallynon-absorbing, optical spectator species at the wavelength λ (405 nm).

TABLE II Ink Compositions. Ink 3 Ink 4 Ink 5 Ink 6 Oligomeric CurableMaterial 36.7 37 35 34.65 Monomeric Curable Material 60 59.7 62.25 62.6Photoinitiator 3.25 3.25 2.7 2.7 Non-Curable Absorber Material 0.05 0.050.05 0.05 D_(p) 71.1 69.4 88.1 85.6 E_(c) 2.5 2.4 4.76 4.48 D_(P)/E_(C)Ratio 28.4 28.9 18.5 19.1

As shown in Table II for Inks 3 and 4, changes in the type of monomericcurable material result in minor changes in D_(p), E_(c), and the ratioD_(p)/E_(c) (and thus also D_(PT)), since the monomeric curablematerials are essentially optical spectator species. Similarly for Inks5 and 6, changes in the type of oligomeric curable material also resultin only minor changes in D_(p), E_(c), and the ratio D_(p)/E_(c).

Example 4 Additional Variations

Inks 7-14 in Table III were prepared according to the procedure inExample 1 (again, the components are provided in Table III as weightpercentages). Table IV identifies the various components used in thevarious Inks in Tables III. In Tables III and IV, dashes (--) indicatethat the component was absent or the value is not reported here.However, to be clear, all of Inks 7-14 are “according to the presentinvention” as broadly described herein (the same is also true for all ofInks 1-6). Additionally, Inks 1-12 are particularly preferredembodiments of the present invention. All components of Inks 7-12 inTable III below, other than the photoinitiator and non-curable absorbermaterial, were substantially non-absorbing at the wavelength λ, suchthat these species were essentially optical spectators, as describedhereinabove.

TABLE III Ink Compositions. Ink 7 Ink 8 Ink 9 Ink 10 Ink 11 Ink 12 Ink13 Ink 14 Oligomeric 55 35 28 26.5 23.85 23.85 18 18 Curable MaterialMonomeric 38 63 69 70 72 73.2 66 66 Curable Material Photoinitiator 51.97 3 3.44 4.1 2.9 4 4 Non-Curable 2 0.03 0.03 0.06 0.05 0.05 2 2Absorber Material Additional — — — — — — 10 10 Component D_(p) — — 84.277.9 96.5 136 17.8 53.3 E_(c) — — 6.95 3.25 4.99 7.12 40.0 54.0D_(P)/E_(C) Ratio — — 12.1 24.0 19.3 19.1 0.445 0.988

TABLE IV Ink Components. Ink 7 Ink 8 Ink 9 Ink 10 Ink 11 Ink 12 Ink 13Ink 14 Oligomeric glycidyl urethane urethane urethane urethane urethaneglycidyl glycidyl Curable acrylate acrylate acrylate acrylate acrylateMaterial Monomeric oxetane/ acrylate acrylate acrylate acrylate acrylateoxetane/ oxetane/ Curable acrylate acrylate acrylate Material (50/50)(50/50) (50/50) Photoinitiator triphenyl Irgacure Irgacure IrgacureIrgacure Irgacure triphenyl triphenyl sulfonium TPO 819 819/ 819/ 819/sulfonium sulfonium Irgacure Irgacure Irgacure 184 184/UVI 184/UVI (7/1)6976 6976 (7/1/3) (4/1/3) Non-Curable pyrene Keystone Keystone KeystoneKeystone Keystone exfoliated fullerene Absorber Oil Oil Oil Oil Oilgraphene Material Yellow Yellow/ Yellow/ Yellow/ Yellow/ KeystoneKeystone Keystone Keystone Blue B Blue B Blue B Blue B (50/50) (50/50)(50/50) (50/50) Additional — — — — — — polyol polyol Component

As illustrated in Table III, different concentrations and types ofphotoinitiators and non-curable absorber materials can be used in inksdescribed herein to adjust D_(p), E_(c), and the ratio D_(p)/E_(c) (andthus also D_(PT)).

Example 5 Comparative Data

Inks 15-17 and Comparative Inks 1 and 2 were prepared according to theprocedure in Example 1. As shown in Table IV, Inks 15 and 16 arecompared with Comparative (“Comp.”) Ink 1. Inks 15 and 16 have the sameoligomeric and monomeric curable material types at comparable amounts asComparative Ink 1. All three inks also include the same photoinitiatorand non-curable absorber material. However, the ratio of photoinitiatorto non-curable absorber material in Comparative Ink 1 is too low toachieve a D_(p)/E_(c) ratio consistent with desired results describedherein (such as printing efficiency and speed), particularly due to thehigh E_(c). Ink 17 and Comparative Ink 2, which are compared to oneanother, include the same oligomeric and monomeric curable materialtypes in comparable amounts. Both inks also include the samephotoinitiator and non-curable absorber material. However, the amount ofnon-curable absorber material in Comparative Ink 2 is too high. Foradditional comparison purposes, it should be further noted that Inks1-12 exhibited improved print through properties while also maintainingdesired mechanical properties, as compared to inks otherwise similar butomitting the combinations of photoinitiator and non-curable absorbermaterial of Inks 1-12 (data not shown).

TABLE IV Comparative Ink Compositions Comp. Comp. Ink 15 Ink 16 Ink 1Ink 17 Ink 2 Oligomeric Curable 57.95 54 56.99 51.9 49 MaterialMonomeric Curable 40 40 42 38 36 Material Photoinitiator 2 5 0.01 10 10Non-Curable Absorber 0.05 1 1 0.1 5 Material D_(p) 70 70 10 25 5 E_(c) 45.6 100 2 100 D_(P)/E_(C) Ratio 18 13 0.1 13 0.05

As demonstrated by the data in the Examples above, it is to beunderstood that inks described and claimed herein are not limited toonly the exact embodiments of Inks 1-17. Instead, based on the teachingsof the present disclosure, other specific inks can be formulated bythose of ordinary skill in the art.

Example 6 Additional Exemplary Ink Compositions

In addition to Inks 1-17 above, other inks according to the presentdisclosure are provided using the amounts in Table V below. The amountsin Table V refer to the wt. % of each component of the identified ink,based on the total weight of the ink, and the total amount equals 100wt. % in a given instance. Additionally, “PI” stands for“photoinitiator.”

TABLE V Ink Components. Oliqomeric Monomeric Non-Curable CurableMaterial Curable Material PI Absorber Material 10-80  0-40  1-10 0.01-210-70 10-40  2-10 0.01-1 20-70  0-60 3-5 0.01-1 20-60 20-60 1-5 0.01-120-50 20-50 2-5  0.01-0.1 20-50 20-70 1-4   0.01-0.05 20-40 50-70 1-4 0.01-0.5 20-50 20-60 2-4 0.01-1 30-40 10-70 2-5 0.01-1 20-40 10-80 1-50.01-1

Some additional, non-limiting example embodiments are provided below.

Embodiment 1. An ink for use in a three-dimensional printing system, theink comprising:

up to 80 wt. % oligomeric curable material;

up to 80 wt. % monomeric curable material;

up to 10 wt. % photoinitiator;

up to 1 wt. % non-curable absorber material; and

up to 10 wt. % one or more additional components, based on the totalweight of the ink,

wherein the total amount of the oligomeric curable material, monomericcurable material, photoinitiator, non-curable absorber material, and oneor more additional components is equal to 100 wt. %;

wherein the photoinitiator is operable to initiate curing of theoligomeric curable material and/or the monomeric curable material whenthe photoinitiator is exposed to incident curing radiation having aGaussian distribution of wavelengths and a peak wavelength λ;

wherein the ink has a penetration depth (D_(p)) and a critical energy(E_(c)) at the wavelength λ; and

wherein the ink has a print through depth (D_(PT)) at the wavelength λof less than or equal to 2×D_(p) and/or a D_(p)/E_(c) ratio of 10-50 (μmcm²)/mJ.

Embodiment 2. The ink of Embodiment 1, wherein the ink has a printthrough depth (D_(PT)) at the wavelength λ of less than or equal to1.5×D_(p).

Embodiment 3. The ink of Embodiment 1 or Embodiment 2, wherein:

the D_(p) of the ink is 60-100 μm; and

the E_(c) of the ink is 2-4 mJ/cm².

Embodiment 4. The ink of Embodiment 1 or Embodiment 2, wherein:

the D_(p) of the ink is 101-150 μm; and

the E_(c) of the ink is 4-20 mJ/cm².

Embodiment 5. The ink of Embodiment 1 or Embodiment 2, wherein:

the D_(p) of the ink is 151-200 μm; and

the E_(c) of the ink is 8-15 mJ/cm².

Embodiment 6. The ink of any of the preceding Embodiments, wherein theink has a ratio of D_(p) to E_(c), in units of (μm cm²)/mJ, of between10 and 50.

Embodiment 7. The ink of any of the preceding Embodiments, wherein theink comprises up to 5 wt. % photoinitiator.

Embodiment 8. The ink of any of the preceding Embodiments, wherein:

the ink comprises up to 5 wt. % photoinitiator and up to 0.5 wt. %non-curable absorber material; and

the ratio of photoinitiator to non-curable absorber material, by weight,is between 5 and 100.

Embodiment 9. The ink of any of the preceding Embodiments, wherein boththe non-curable absorber material and the photoinitiator have anabsorption peak within 30 nm of the wavelength λ.

Embodiment 10. The ink of any of the preceding Embodiments, wherein thetotal absorbance of the non-curable absorber material at the wavelengthλ is about 0.1 to 10 times the total absorbance of the photoinitiator atthe wavelength λ.

Embodiment 11. The ink of any of the preceding Embodiments, wherein thenon-curable absorber material comprises pyrene.

Embodiment 12. The ink of any of the preceding Embodiments, wherein thenon-curable absorber material comprises an oil-soluble yellow dye.

Embodiment 13. A method of forming a three-dimensional article byadditive manufacturing, the method comprising:

providing the ink of any of Embodiments 1-12; and

selectively curing a portion of the ink using incident curing radiationhaving a Gaussian distribution of wavelengths and a peak wavelength atthe wavelength λ.

Embodiment 14. The method of Embodiment 13, wherein:

the ink is selectively cured according to preselected computer aideddesign (CAD) parameters; and

the D_(p) corresponds to a voxel depth of the CAD parameters.

Embodiment 15. The method of Embodiment 13 or Embodiment 14, whereinproviding the ink comprises selectively depositing layers of the ink ina fluid state onto a substrate to form the three-dimensional article.

Embodiment 16. The method of any of Embodiments 13-15, wherein:

providing the ink comprises retaining the ink in a fluid state in acontainer;

selectively curing a portion of the ink comprises selectively applyingthe curing radiation to the ink in the container to solidify at least aportion of a first fluid layer of the ink, thereby forming a firstsolidified layer that defines a first cross-section of the article;

raising or lowering the first solidified layer to provide a second fluidlayer of the ink at a surface of the fluid ink in the container; and

selectively applying the curing radiation to the ink in the container tosolidify at least a portion of the second fluid layer of the ink,thereby forming a second solidified layer that defines a secondcross-section of the article, the first cross-section and the secondcross-section being bonded to one another in a z-direction.

Embodiment 17. A printed three-dimensional article formed from the inkof any of Embodiments 1-12 and/or using the method of any of Embodiments13-16.

All patent documents referred to herein are incorporated by reference intheir entireties. Various embodiments of the invention have beendescribed in fulfillment of the various objectives of the invention. Itshould be recognized that these embodiments are merely illustrative ofthe principles of the present invention. Numerous modifications andadaptations thereof will be readily apparent to those skilled in the artwithout departing from the spirit and scope of the invention.

That which is claimed:
 1. A method of forming a three-dimensionalarticle by additive manufacturing, the method comprising: providing anink; and selectively curing a portion of the ink using incident curingradiation having a Gaussian distribution of wavelengths and a peakwavelength at a wavelength λ, wherein the ink comprises up to 80 wt. %oligomeric curable material, based on the total weight of the ink; up to80 wt. % monomeric curable material, based on the total weight of theink; up to 10 wt. % photoinitiator, based on the total weight of theink; up to 1 wt. % non-curable absorber material, based on the totalweight of the ink; and up to 10 wt. % one or more additional components,based on the total weight of the ink, wherein the total amount of theoligomeric curable material, monomeric curable material, photoinitiator,non-curable absorber material, and one or more additional components isequal to 100 wt. %; wherein the photoinitiator is operable to initiatecuring of the oligomeric curable material and/or the monomeric curablematerial when the photoinitiator is exposed to the incident curingradiation; wherein the ink has a penetration depth (D_(p)) and acritical energy (E_(c)) at the wavelength λ, and the ratio of D_(p) toE_(c), in units of (μm cm²)/mJ, is between 10 and 50; and wherein theink has a print through depth (D_(PT)) at the wavelength λ of less thanor equal to 2×D_(p).
 2. The method of claim 1, wherein: the ink isselectively cured according to preselected computer aided design (CAD)parameters; and the D_(p) corresponds to a voxel depth of the CADparameters.
 3. The method of claim 1, wherein providing the inkcomprises selectively depositing layers of the ink in a fluid state ontoa substrate to form the three-dimensional article.
 4. The method ofclaim 1, wherein: providing the ink comprises retaining the ink in afluid state in a container; selectively curing a portion of the inkcomprises selectively applying the curing radiation to the ink in thecontainer to solidify at least a portion of a first fluid layer of theink, thereby forming a first solidified layer that defines a firstcross-section of the article; raising or lowering the first solidifiedlayer to provide a second fluid layer of the ink at a surface of thefluid ink in the container; and selectively applying the curingradiation to the ink in the container to solidify at least a portion ofthe second fluid layer of the ink, thereby forming a second solidifiedlayer that defines a second cross-section of the article, the firstcross-section and the second cross-section being bonded to one anotherin a z-direction.
 5. The method of claim 1, wherein the E_(c) of the inkis 2-4 mJ/cm².
 6. The method of claim 1, wherein the E_(c) of the ink is4-20 mJ/cm².
 7. The method of claim 1, wherein the D_(p) of the ink is151-200 μm.
 8. The method of claim 1, wherein the ink comprises up to 5wt. % photoinitiator.
 9. The method of claim 1, wherein: the inkcomprises up to 5 wt. % photoinitiator and up to 0.5 wt. % non-curableabsorber material; and the ratio of photoinitiator to non-curableabsorber material, by weight, is between 5 and
 100. 10. The method ofclaim 1, wherein both the non-curable absorber material and thephotoinitiator have an absorption peak within 30 nm of the wavelength λ.11. The method of claim 1, wherein the total absorbance of thenon-curable absorber material at the wavelength λ is about 0.1 to 10times the total absorbance of the photoinitiator at the wavelength λ.12. The method of claim 1, wherein the non-curable absorber materialcomprises a polycyclic aromatic compound.
 13. The method of claim 1,wherein the non-curable absorber material comprises pyrene.
 14. Themethod of claim 1, wherein the non-curable absorber material comprisesan oil-soluble yellow dye.
 15. The method of claim 1, wherein thephotoinitiator comprises an alpha-cleavage type photoinitiator.
 16. Themethod of claim 1, wherein the oligomeric curable material comprises(meth)acrylate moieties.
 17. The method of claim 1, wherein themonomeric curable material comprises (meth)acrylate moieties.
 18. Aprinted three-dimensional article formed from an ink comprising: up to80 wt. % oligomeric curable material, based on the total weight of theink; up to 80 wt. % monomeric curable material, based on the totalweight of the ink; up to 10 wt. % photoinitiator, based on the totalweight of the ink; up to 1 wt. % non-curable absorber material, based onthe total weight of the ink; and up to 10 wt. % one or more additionalcomponents, based on the total weight of the ink, wherein the totalamount of the oligomeric curable material, monomeric curable material,photoinitiator, non-curable absorber material, and one or moreadditional components is equal to 100 wt. %; wherein the photoinitiatoris operable to initiate curing of the oligomeric curable material and/orthe monomeric curable material when the photoinitiator is exposed toincident curing radiation having a Gaussian distribution of wavelengthsand a peak wavelength λ; wherein the ink has a penetration depth (D_(p))and a critical energy (E_(c)) at the wavelength λ, and the ratio ofD_(p) to E_(c), in units of (μm cm²)/mJ, is between 10 and 50; andwherein the ink has a print through depth (D_(PT)) at the wavelength λof less than or equal to 2×D_(p).